A wireless mobile communication system such as a long term evolution (LTE) system includes a single downlink and a single uplink. In other words, a single uplink carrier and a single downlink carrier exist to correspond to each other. Also, in the LTE system, since one of downlink and uplink carriers designated by a base station is selected as a carrier used by a mobile station, it is possible to definitely identify what carrier is used to transmit and receive a signal. However, an IMT-advanced system that provides a multi-carrier environment supports a wider communication frequency band by extending a single carrier system of the related art, and considers a method for supporting higher throughput based on the wider communication frequency band. Under the circumstances, a problem occurs in carrier identity, which has not occurred when the single uplink/downlink carrier of the related art has been used. The problem of carrier identity means a problem of identity as to which one of a plurality of downlink carriers is used by a mobile station as illustrated in FIG. 1 below.
Referring to FIG. 1, it is assumed that two carriers DL1 and DL2 for a downlink exist and a single carrier UL1 for an uplink exists. FIG. 1 illustrates a problem that occurs when a base station responds to random access performed by a mobile station in a state that two downlink carriers DL1 and DL2 are a single uplink carrier UL1 are aggregated. The base station can transmit information of the system by using the first downlink carrier DL1 and/or the second downlink carrier DL2. In this case, if the mobile station performs cell search through the first downlink carrier DL1 and receives the information of the system, it can obtain information on frequency and band of the uplink corresponding to the first downlink carrier DL1 when the information of the system is the same as that of the single carrier system according to the related art. Accordingly, the mobile station transmits a random access preamble in accordance with random access configuration obtained through the downlink by using the corresponding uplink. When receiving the random access preamble, the base station can identify that the mobile station accesses the system by detecting the preamble. However, since the base station cannot identify which one of the first downlink carrier DL1 and the second downlink carrier DL2 has been used by the corresponding mobile station to access the system, a problem occurs in that it is difficult for the base station to use which one of the first downlink carrier DL1 and the second downlink carrier DL2 to respond to the mobile station. In other words, a problem occurs in that ambiguity occurs in an initial access process.